Lens system, interchangeable lens apparatus, and camera system

ABSTRACT

It has conventionally been difficult to reduce the size of a fast lens system having an F number of about 1.4 to 2.4. In a lens system of the present invention, a positive lens element is disposed closest to an object side. A diaphragm is disposed in a widest air space in the lens system. The lens system of the present invention satisfies the following conditions:
 
0.05&lt; L _1/ L   —   TH &lt;0.21
 
1.5&lt; L   —   TH/Y&lt; 8
         where   L_1 is an interval from a lens surface located closest to the object side to a lens surface located on the object side relative to the diaphragm;   L_TH is an interval from the lens surface located closest to the object side to a lens surface located closest to an image side; and   Y is a maximum image height.

This application is a division of U.S. patent application Ser. No.12/869,957 filed on Aug. 27, 2010 and issued as U.S. Pat. No. 8,400,719on Mar. 19, 2013, which is incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a single-focal-length imaging lenssystem that is applicable to an imaging optical system, particularly, aninterchangeable lens or a digital still camera.

2. Description of the Background Art

With the growing popularity of cameras, lenses of various specificationshave been proposed. Prior art documents proposing lens systems having Fnumbers ranging from about 1.4 to 2.4 are as follows: Japanese Laid-OpenPatent Publication No. 63-96619, Japanese Laid-Open Patent PublicationNo. 2-23309, Japanese Laid-Open Patent Publication No. 8-171050,Japanese Laid-Open Patent Publication No. 2000-19393, and JapaneseLaid-Open Patent Publication No. 2002-250863.

SUMMARY OF THE INVENTION

Hereinafter, the above prior art documents and problems thereof will bedescribed.

In the lens system disclosed in Japanese Laid-Open Patent PublicationNo. 63-96619, a lens unit disposed before a diaphragm is constituted bya single lens, whereas a lens unit disposed after the diaphragm is verylarge in size, resulting in an increase in the total length of theentire lens system.

In the lens system disclosed in Japanese Laid-Open Patent PublicationNo. 2-23309, a chromatic aberration is actively compensated by using aGRIN lens. Although a diaphragm, which is increased in size when used ina large-diameter lens system, is disposed in a widest air space in thelens system, the distance from a lens surface closest to an object sideto the space where the diaphragm is disposed is long, resulting in anincrease in the size of the lens system.

In the lens system disclosed in Japanese Laid-Open Patent Publication No8-171050, two GRIN lenses are used to reduce the number of lenselements. This lens system is reduced in size by disposing the two GRINlenses in a space from an object to a diaphragm. However, the length ofa lens unit from a lens surface closest to an object side to a lenssurface located on the object side relative to a diaphragm is increasedrelative to the total length of the lens system, resulting in anincrease in the total length of the lens system.

A GRIN lens is a lens for compensating an aberration by utilizinggradient index distribution inside the lens. However, it is difficult tomanufacture a base material thereof prior to processing, as comparedwith a spherical lens or an aspheric lens. Accordingly, a lens systemhaving such GRIN lens is currently not popular.

In the lens systems disclosed in Japanese Laid-Open Patent PublicationNo. 2000-19393 and Japanese Laid-Open Patent Publication No.2002-250863, although a diaphragm, which is increased in size when usedin a large-diameter lens system, is disposed in a widest air space inthe lens system, the entire lens system is significantly increased insize.

An object of the present invention is to provide a compact and fast lenssystem having an F number of about 1.4 to 2.4.

A lens system according to the present invention is suitable for sizereduction of a fast lens system having an F number of about 1.4 to 2.4,in which a lens element closest to an object side is a positive lenselement and a diaphragm is disposed in a widest air space in the lenssystem. The lens system satisfies the following conditions:0.05<L _(—)1/L _(—) TH<0.211.5<L _(—) TH/Y<8

where,

L_(—)1 is an interval from a lens surface located closest to the objectside to a lens surface located on the object side relative to thediaphragm (a length of a front unit including all lens elements that aredisposed on the object side relative to the diaphragm),

L_TH is an interval from the lens surface located closest to the objectside to a lens surface located closest to an image side, and

Y is a maximum image height.

Further, a lens system according to the present invention has an Fnumber equal to or smaller than 2.4, and includes two or more lenselements each having a refractive index equal to or greater than 1.85.The lens system satisfies the following condition:1.5<L _(—) TH/Y<8

According to the present invention, it is possible to provide a compactand fast lens system having an F number of about 1.4 to 2.4.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram according to Embodiment 1 (Example 1), wherein (a)shows a lens arrangement diagram, and (b) shows longitudinal aberrationdiagrams;

FIG. 2 is a diagram according to Embodiment 2 (Example 2), wherein (a)shows a lens arrangement diagram, and (b) shows longitudinal aberrationdiagrams;

FIG. 3 is a diagram according to Embodiment 3 (Example 3), wherein (a)shows a lens arrangement diagram, and (b) shows longitudinal aberrationdiagrams;

FIG. 4 is a diagram according to Embodiment 4 (Example 4), wherein (a)shows a lens arrangement diagram, and (b) shows longitudinal aberrationdiagrams;

FIG. 5 is a diagram according to Embodiment 5 (Example 5), wherein (a)shows a lens arrangement diagram, and (b) shows longitudinal aberrationdiagrams;

FIG. 6 is a diagram according to Embodiment 6 (Example 6), wherein (a)shows a lens arrangement diagram, and (b) shows longitudinal aberrationdiagrams;

FIG. 7 is a diagram according to Embodiment 7 (Example 7), wherein (a)shows a lens arrangement diagram, and (b) shows longitudinal aberrationdiagrams;

FIG. 8 is a diagram according to Embodiment 8 (Example 8), wherein (a)shows a lens arrangement diagram, and (b) shows longitudinal aberrationdiagrams;

FIG. 9 is a diagram according to Embodiment 9 (Example 9), wherein (a)shows a lens arrangement diagram, and (b) shows longitudinal aberrationdiagrams;

FIG. 10 is a diagram according to Embodiment 10 (Example 10), wherein(a) shows a lens arrangement diagram, and (b) shows longitudinalaberration diagrams;

FIG. 11 is a diagram according to Embodiment 11 (Example 11), wherein(a) shows a lens arrangement diagram, and (b) shows longitudinalaberration diagrams; and

FIG. 12 is a diagram illustrating a schematic configuration of aninterchangeable-lens type digital camera system according to Embodiment12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIGS. 1 to 11, (a) shows a lens arrangement diagram, and (b) showslongitudinal aberration diagrams in an infinity in-focus condition. Ineach lens arrangement diagram (a), an asterisk “*” imparted to aparticular surface indicates that the surface is aspheric. A straightline located on the most right-hand side indicates the position of animage surface S. A symbol A in the lens system indicates a diaphragm.Lens elements constituting the lens system are given numbers L1, L2, . .. in order from a lens element located closest to an object side to alens element located closest to an image side.

Further, in the present specification, a lens unit from a lens surfacelocated closest to the object side to a lens surface located on theobject side relative to the diaphragm is referred to as a front unit,and a lens unit from a lens surface located on the image side relativeto the diaphragm to a lens surface located closest to the image side isreferred to as a rear unit.

Hereinafter, preferred embodiments of the present invention will bedescribed with reference to the drawings.

Each of lens systems according to Embodiments 1 to 11 of the presentinvention has an F number of 1.4 to 2.4, and includes, in order from theobject side toward the image side, a front unit including two lenselements, i.e., a positive lens element and a negative lens element, anda rear unit having a positive power. The rear unit includes at least onenegative lens element and at least one aspheric surface. A diaphragm Ais disposed in a widest air space in the lens system.

Embodiment 1

A lens system shown in FIG. 1 has a front unit that includes, in orderfrom the object side toward the image side, a spherical positive lenselement L1 and a negative lens element L2 having an aspheric surface onits object side. A rear unit having a positive power is disposed at theobject side relative to the front unit. The rear unit includes twonegative lens elements and three positive lens elements. Morespecifically, the rear unit includes, in order from the object sidetoward the image side, a cemented lens element including a negative lenselement L3 and a positive lens element L4, a cemented lens elementincluding a positive lens element L5 and a negative lens element L6, anda positive lens element L7 having a refractive index of 1.69 and havingan aspheric surface on its image side. At the time of focusing from aninfinity in-focus condition to a close-point in-focus condition, theentire lens system moves toward the object side along the optical axis.

Embodiment 2

A lens system shown in FIG. 2 has a front unit including, in order fromthe object side toward the image side, a positive lens element L1 havingan aspheric surface, and a spherical negative lens element L2 having anegative power. An aspheric lens element L7 is disposed closest to theimage side.

Embodiment 3

A lens system shown in FIG. 3 has a front unit including, in order fromthe object side toward the image side, a positive lens element L1 havingan aspheric surface, and a spherical negative lens element L2 having arefractive index of 1.49. A lens element L7 disposed closest to theimage side is a both-side aspheric lens element.

Embodiment 4

A lens system shown in FIG. 4 includes, in order from the object sidetoward the image side, a positive lens element L1 and a negative lenselement L2. An air space between the periphery of the positive lenselement L1 and the periphery of the negative lens element L2 is narrowerthan an air space between the center of the positive lens element L1 andthe center of the negative lens element L2. The negative lens element L2has a concave surface on its object side, and the positive lens elementL1 and the negative lens element L2 can be fixed in a state where acircumferential portion of the negative lens element L2 facing theobject side contacts a surface of the positive lens element L1 facingthe image side. A lens element L7 disposed closest to the image side isa both-side aspheric lens element having a high refractive index of1.88.

Embodiment 5

In a lens system shown in FIG. 5, a positive lens element L1 included ina front unit has a low refractive index of 1.73. Further, an air spacebetween the periphery of a lens element L6 and the periphery of a lenselement L7 is wider than an air space between the center of the lenselement L6 and the center of the lens element L7

Embodiment 6

In a lens system shown in FIG. 6, a positive lens element L1 included ina front unit has a high refractive index of 1.95. A rear unit includesfour lens elements (two negative lens elements and two positive lenselements). A lens element L6 closest to the image side is a sphericallens element, and a lens element L5 second closest to the image side isan aspheric lens element.

Embodiment 7

In a lens system shown in FIG. 7, a front unit is constituted byspherical lens elements. A positive lens element L1 and a negative lenselement L2 in the front unit are cemented with each other. A rear unitincludes one negative lens element and three positive lens elements. Alens element L7 closest to the image side is a spherical lens element,and a lens element L5 second closest to the image side is an asphericlens element.

Embodiment 8

In a lens system shown in FIG. 8, a front unit is constituted byspherical lens elements. An air space is provided between a positivelens element L1 and a negative lens element L2.

Embodiment 9

A lens system shown in FIG. 9 is a fast lens system having an F numberof about 1.7, and a half view angle increased to 37.6 degrees. Anaspheric surface is provided in a front unit.

Embodiment 10

A lens system shown in FIG. 10 is a fast lens system having an F numberof about 1.7, and a half view angle increased to 37.6 degrees. A frontunit is constituted by spherical lens elements.

Embodiment 11

In a lens system shown in FIG. 11, both surfaces of a lens element L7closest to the image side are aspheric surfaces. The lens system has anF number of 1.4. A positive lens element having a refractive indexexceeding 1.9 is disposed in a rear unit, whereby an astigmaticdifference is favorably corrected even when the F number is large.

The lens systems of the respective embodiments of the present inventionare fast lens systems having the F numbers ranging from 1.4 to 2.1. Thepositive lens element disposed closest to the object side suppresses anincrease in the light beam diameter of the lens unit disposed on theimage side relative to the lens element closest to the object side. Thatis, the positive lens element suppresses retrofocus characteristics.Accordingly, the diaphragm mechanism provided between the lens surfacescan be reduced in size. Further, since the diaphragm mechanism isdisposed in the widest air space in the lens system, an expansion of thespace occupied by the diaphragm mechanism can be suppressed. The lenssystems of the respective embodiments of the present invention satisfythe following conditional expressions (1) and (2):0.05<L _(—)1/L _(—) TH<0.21  (1)1.5<L _(—) TH/Y<8  (2)

where,

L_(—)1 is an interval from a lens surface located closest to the objectside to a lens surface located on the object side relative to thediaphragm in the lens system (the length of the front unit that is alens unit disposed on the object side relative to the diaphragm).

L_TH is an interval from the lens surface located closest to the objectside to the lens surface located closest to the image side in the lenssystem, and

Y is a maximum image height.

Conditional expressions (1) and (2) provide the conditions for sizereduction of the lens system. Specifically, the condition (1) sets forththe thickness of the front unit. When the value exceeds the upper limitof the condition (1), the thickness of the front unit is increased, andthe front lens diameter is also increased, resulting in an increase inthe size of the lens system. When the value goes below the lower limitof the condition (1), the thickness of the front unit is excessivelydecreased, resulting in a difficulty in the lens arrangement.Preferably, the lower limit of the condition (1) is set to 0.09 tofacilitate the lens arrangement in the front unit.

The condition (2) sets forth the distance from the lens surface closestto the object side to the lens surface closest to the image side. A lenssystem in which the value exceeds the upper limit of the condition (2)is not small in size, whereas a lens system in which the value goesbelow the lower limit of the condition (2) causes a difficulty inarranging lens elements and a diaphragm mechanism. Preferably, the upperlimit of the condition (2) is about 5 and the lower limit thereof isabout 2, thereby obtaining a small-sized lens system having a favorableaberration.

Further, a glass material having a high refractive index not less than1.85 is used for the two or more lens elements constituting the lenssystem to reduce the sizes of the lens elements.

When the front unit is constituted by two lens elements, i.e., apositive lens element and a negative lens element, in order from theobject side, an astigmatic difference. Which is hard to correct by usingone positive lens element, can be easily corrected. Further, since thelight beam diameter can be reduced as compared with the case where anegative lens element is disposed closest to the object side, sizereduction of the lens system is facilitated.

Further, when two or more positive lens elements are disposed in therear unit to disperse the positive power of the rear unit into therespective positive lens elements, the curvature radius of each of thesurfaces of the positive lens elements can be increased. Thereby anastigmatic difference, which is likely to occur in a fist lens having alarge F number, can be suppressed within a range from the maximum imageheight to the intermediate image height.

Furthermore, when, in the rear unit, an aspheric surface is provided onthe lens element closest to the image side or on the lens element secondclosest to the image side, a field curvature that occurs in the rearunit can be easily corrected. Preferably, the lens surface closest tothe image side is aspheric. More preferably, the both surfaces of thelens element closest to the image side are aspheric, respectively.

Furthermore, effective correction of an astigmatic difference isrealized when a lens element having a lens surface of a smallestcurvature radius in the rear unit is a negative lens element, a lenselement having a second smallest curvature radius is a positive lenselement, and the negative lens element and the positive lens element areadjacent to each other. The negative lens element having the surface ofthe smallest curvature radius and the positive lens element having thesurface of the second smallest curvature radius cancel an astigmaticdifference. If the distance between these two surfaces is increased,astigmatic difference correction over the entire image surface becomesimbalanced, resulting in a difficulty in flattening the image surface inthe entire optical system.

The refractive index NdL1 of the positive lens element disposed closestto the object side satisfies the following condition:1.70<NdL1<2.4  (3)

where NdL1 is the refractive index of the positive lens element,disposed closest to the object side.

The condition (3) sets forth the refractive index of the positive lenselement disposed closest to the object side. If the value goes below thelower limit of the condition (3), the Petzval sum becomes excessivelysmall. Thus, this situation is unpreferable. When the value of NdL1 is1.8 or more, flattening of the image surface is facilitated. If thevalue exceeds the upper limit of the condition (3), the Petzval sumbecomes excessively large. Thus, this situation is unpreferable. Whenthe value of NdL1 is 2.1 or less, flattening of the image surface isfacilitated.

The refractive indices of the positive lens element and the negativelens element constituting the front unit satisfy the followingcondition:0.20<NdL1−NdL2<0.45  (4)

where NdL2 is the refractive index of the negative lens element disposedsecond closest to the object side.

When the value exceeds the upper limit of the condition (4), a fieldcurvature toward the under side occurs due to an air lens providedbetween the two lens elements in the front unit. When the value goesbelow the lower limit of the condition (4), a field curvature toward theover side occurs, which is difficult to correct.

Further, in the lens system, a distortion at the maximum image heightsatisfies the following condition:−16%<Dist.<0%  (5)

where Dist is the distortion at the maximum image height.

The condition (5) sets forth the distortion. When the value exceeds theupper limit of the condition (5), the power of the positive lens elementin the front unit, in which the height of an off-axis beam is increased,is undesirably increased, and the image surface is apt to fall towardthe under side. When the value goes below the lower limit of thecondition (5), a strong barrel distortion occurs, which is difficult tocorrect.

While each of the lens systems of Embodiments 1 to 11 includes two lenselements in the front unit, the front unit may include three lenselements. Further, the diaphragm may be disposed close to the lenssurface on the image side in the widest air space, and thus the rearunit can be easily reduced in size.

Some of the lens systems of Embodiments 1 to 11 have no aspheric surfacein the front unit while the other lens systems have one aspheric surfacein the front unit. However, a lens system having two or more asphericsurfaces in a front unit is also within the scope of the presentinvention.

Further, a lens system having three or more aspheric surfaces in a rearunit is also within the scope of the present invention.

As shown in Numerical Examples 1 to 4 and 6 to 11 described below, sizereduction of a lens system is easily realized when the lens systemsatisfies both the conditions (1) and (2), or the condition (3), andincludes three or more lens elements each having a refractive indexequal to or greater than 1.80. Particularly, size reduction is realizedmore easily if one or more lens elements in the front unit and two ormore lens elements in the rear unit have refractive indices equal to orgreater than 1.80.

The power of the front unit of the lens system satisfies the followingcondition:−0.35<φL1/φ_(—) L<0.35  (6)

where,

φ_L1 is the power of the front unit, and

φ_L1 is the power of the rear unit.

When the value goes below the lower limit of the condition (6), thenegative power of the front unit is excessively increased, and theentire lens system exhibits significant retrofocus characteristics,resulting in an increase in the air space where the subsequent diaphragmis disposed and/or an increase in the size of the rear unit. When thevalue exceeds the upper limit of the condition (6), the power of thefront unit is increased, and the curvature radius of each of the lenssurfaces in the front unit is excessively reduced, resulting in adifficulty in correcting an astigmatic difference.

The lens units constituting the zoom lens system according to any of theembodiments of the present invention may be composed exclusively ofrefractive type lens elements that deflect the incident light byrefraction (that is, lenses of a type in which deflection is achieved atthe interface between media each having a distinct refractive index).Alternatively, the lens units may employ any one of or a combination ofsome of: diffractive type lens elements that deflect the incident lightby diffraction; refractive-diffractive hybrid type lens elements thatdeflect the incident light by a combination of diffraction andrefraction; and gradient index type lens elements that deflect theincident light by distribution of refractive index in the medium.

Although not specifically described in the embodiments of the presentinvention, a parallel plate having no substantial power such as anoptical low-pass filter or a face plate of an image sensor, or amicrolens array for increasing the aperture efficiency of the imagesensor may be provided between the image surface S and the lens system.

Embodiment 12

FIG. 12 is a schematic construction diagram showing aninterchangeable-lens type digital camera system according to Embodiment12. An interchangeable-lens type digital camera system 100 (hereinafter,simply referred to as “camera system”) according to the presentembodiment includes a camera body 101, and an interchangeable lensapparatus 201 connected to the camera body 101 in an attachable andremovable manner.

The camera body 101 includes an image sensor 102 which receives anoptical image formed by a lens system 202 of the interchangeable lensapparatus 201 to convert the optical image into an electric imagesignal, a liquid crystal display monitor 103 which displays the imagesignal obtained by the image sensor 102, and a camera mount section 104.On the other hand, the interchangeable lens apparatus 201 includes thelens system 202 according to any of Embodiments 1 to 11, a lens barrelwhich holds the lens system 202, and a lens mount section 204 connectedto the camera mount section 104 of the camera body. The camera mountsection 104 and the lens mount section 204 are physically coupled toeach other. Further, the camera mount section 104 and the lens mountsection 204 function as interfaces for electrically connecting acontroller (not shown) inside the camera body 101 and a controller (notshown) inside the interchangeable lens apparatus 201 to achieve mutualsignal communication.

The camera system 100 of the present embodiment includes the lens system202 according to any of Embodiments 1 to 11, and hence is capable ofobtaining a preferable optical image.

EXAMPLES

Hereinafter, numerical examples will be described, in which the lenssystems according to Embodiments 1 to 11 are implemented specifically.Numerical Examples 1 to 11 corresponds to Embodiments 1 to 11,respectively. In each numerical example, the units of the length in thetable are all “mm”, and the units of the view angle are all “°”.Further, r is the radius of curvature, d is the axial distance, nd isthe refractive index to the d-line, and vd is the Abbe number to thed-line. Moreover, the surfaces marked with “*” are aspheric surfaces,and the aspheric surface configuration is defined by the followingformula:

$Z = {\frac{h^{2}/r}{1 + \sqrt{1 - {\left( {1 + \kappa} \right)\left( {h/r} \right)^{2}}}} + {\sum{A_{n}h^{n}}}}$

Here, the symbols in the formula indicate the following quantities:

Z is the distance from an on-the-aspheric-surface point at a height hrelative to the optical axis to a tangential plane at the top of theaspheric surface;

h is the height relative to the optical axis;

r is the radius of curvature at the top;

κ is the conic constant; and

An is the n-th order aspheric coefficient.

The longitudinal aberration diagrams shown in each of FIGS. 1( b) to11(b) show, in order from the left-hand side, the spherical aberration(SA(mm)), the astigmatism (AST (mm)), and the distortion (DIS (%)). Ineach spherical aberration diagram, the horizontal axis indicates thedefocus amount, the vertical axis indicates the F-number (in eachdiagram, indicated as and the solid line, the short dash line, and thelong dash line indicate the characteristics to the d-line, the F-line,and the C-line, respectively. In each astigmatism diagram, thehorizontal axis indicates the defocus amount, the vertical axisindicates the image height (in each diagram, indicated as “H”), and thesolid line and the dash line indicate the characteristics to thesagittal image plane (in each diagram, indicated as “s”) and themeridional image plane (in each diagram, indicated as “m”),respectively. In each distortion diagram, the horizontal axis indicatesthe distortion, and the vertical axis indicates the image height (ineach diagram, indicated as “H”).

Numerical data of Numerical Examples 1 to 11 are shown in Tables 1 to11, respectively. Each table includes surface data, aspheric surfacedata, and various data.

(Numerical Example 1)

TABLE 1 Surface data Surface number r d nd vd Object surface ∞  124.61620 2.83620 1.89292 35.6  2 49.69520 0.20000  3 31.01750 1.000001.51788 70.1  4* 11.98740 11.18170  5(Diaphragm) ∞ 4.07000  6 −9.397400.70000 1.74250 27.2  7 26.34660 0.01000 1.56732 42.8  8 26.346603.99780 1.88300 40.8  9 −14.56140 0.20000 10 18.44590 5.16480 1.8830040.8 11 −23.96990 0.01000 1.56732 42.8 12 −23.96990 0.70000 1.64642 33.013 15.69950 1.19040 14 25.19460 2.51300 1.68863 52.9 15* −63.49530 BFImage surface ∞ Aspherical data Surface No. Parameters  4 K =0.00000E+00, A4 = 7.34318E−06, A6 = 5.87040E−08, A8 = 0.00000E+00 15 K =−1.00000E+00, A4 = 8.97198E−05, A6 = 1.94080E−08, A8 = 4.82039E−09Various data Focal length 17.4874 F-number 1.75427 View angle 32.3373Image height 10.3000 Overall length of lens system 49.9962 BF 16.22225

(Numerical Example 2)

TABLE 2 Surface data Surface number r d nd vd Object surface ∞  1*26.40870 2.98450 1.89400 35.1  2 68.84630 0.20000  3 39.28910 1.000001.51788 70.1  4 11.65210 11.18200  5(Diaphragm) ∞ 4.07000  6 −9.383900.70000 1.75018 26.9  7 27.94700 0.01000 1.56732 42.8  8 27.947003.50550 1.88300 40.8  9 −14.13990 0.20000 10 18.83350 5.49620 1.8830040.8 11 −22.79060 0.01000 1.56732 42.8 12 −22.79060 0.70000 1.63911 33.613 15.91690 1.13030 14 24.76660 2.58600 1.68863 52.9 15* −56.99360 BFImage surface ∞ Aspherical data Surface No. Parameters  1 K =0.00000E+00, A4 = −4.57047E−06, A6 = 1.32032E−09, A8 = 0.00000E+00 15 K= −1.00000E+00, A4 = 9.04831E−05, A6 = 3.96224E−08, A8 = 4.67740E−09Various data Focal length 16.9873 F-number 1.76534 View angle 32.9941Image height 10.2000 Overall length of lens system 49.9955 BF 16.22100

(Numerical Example 3)

TABLE 3 Surface data Surface number r d nd vd Object surface ∞  118.33950 1.61230 1.84666 23.8  2* 24.12010 0.20000  3 12.54790 1.000001.48749 70.4  4 8.25590 5.35210  5(Diaphragm) ∞ 3.68960  6 −8.065500.70000 1.73931 27.4  7 61.97230 0.01000 1.56732 42.8  8 61.972302.66950 1.88300 40.8  9 −13.06140 0.20000 10 19.96690 4.44040 1.8830040.8 11 −14.43760 0.01000 1.56732 42.8 12 −14.43760 0.70000 1.66436 31.613 14.78390 1.23620 14* 26.47990 2.68070 1.81195 45.6 15* −35.90830 BFImage surface ∞ Aspherical data Surface No. Parameters  2 K =0.00000E+00, A4 = −5.46482E−06, A6 = −1.05369E−07, A8 = 1.74342E−10 14 K= 0.00000E+00, A4 = 5.33899E−05, A6 = 4.21138E−07, A8 = 8.84863E−09 15 K= −1.03675E+00, A4 = 1.18333E−04, A6 = 2.68454E−07, A8 = 1.96446E−08Various data Focal length 17.0000 F-number 1.75434 View angle 33.0735Image height 10.2000 Overall length of lens system 40.9999 BF 16.49910

(Numerical Example 4)

TABLE 4 Surface data Surface number r d nd vd Object surface ∞  125.90510 3.02210 1.89635 34.1  2 −67.53030 0.62230  3 −31.75050 1.000001.57678 41.1  4* 28.62600 3.09450  5(Diaphragm) ∞ 3.64910  6 −8.317500.70000 1.75447 26.7  7 17.54630 0.01000 1.56732 42.8  8 17.546302.47620 1.88300 40.8  9 −28.62110 0.20000 10 20.64260 3.79470 1.8830040.8 11 −16.59530 0.01000 1.56732 42.8 12 −16.59530 0.70000 1.62169 35.313 16.62870 0.96460 14* 21.47920 2.99900 1.88300 40.8 15* −36.41170 BFImage surface ∞ Aspherical data Surface No. Parameters  4 K =0.00000E+00, A4 = −8.40576E−05, A6 = −1.19409E−06, A8 = 0.00000E+00 14 K= 0.00000E+00, A4 = 5.76001E−06, A6 = 2.37507E−07, A8 = 3.28130E−09 15 K= −1.00000E+00, A4 = 1.10937E−04, A6 = 3.09430E−07, A8 = 6.96800E−09Various data Focal length 19.9991 F-number 1.75993 View angle 28.8697Image height 10.5000 Overall length of lens system 40.0067 BF 16.76425

(Numerical Example 5)

TABLE 5 Surface data Surface number r d nd vd Object surface ∞  116.64440 2.77040 1.72916 54.7  2* 40.88740 0.20000  3 19.54880 1.000001.48749 70.4  4 7.99180 5.45480  5(Diaphragm) ∞ 3.53590  6 −7.703000.70000 1.61841 35.6  7 16.96980 0.01000 1.56732 42.8  8 16.969803.75830 1.88300 40.8  9 −14.89970 0.20000 10 41.58130 3.46160 1.8830040.8 11 −14.01750 0.01000 1.56732 42.8 12 −14.01750 0.70000 1.78186 25.713 37.44260 0.20000 14* 41.69860 2.50000 1.79656 46.9 15* −26.14760 BFImage surface ∞ Aspherical data Surface No. Parameters  2 K =0.00000E+00, A4 = 9.48672E−06, A6 = −1.25461E−07, A8 = 5.54880E−10 14 K= 0.00000E+00, A4 = 5.69581E−05, A6 = 4.22989E−07, A8 = 1.28103E−08 15 K= −1.64452E+00, A4 = 1.26186E−04, A6 = 4.12313E−07, A8 = 2.08508E−08Various data Focal length 16.7073 F-number 1.75971 View angle 33.5429Image height 10.2000 Overall length of lens system 40.9970 BF 16.49601

(Numerical Example 6)

TABLE 6 Surface data Surface number r d nd vd Object surface ∞  121.94370 2.37910 1.95000 45.0  2 47.72830 0.20000  3 15.41910 2.249201.68893 31.1  4* 11.33210 4.73570  5(Diaphragm) ∞ 3.90880  6 −9.221500.70000 1.60000 30.0  7 20.38040 0.01000 1.56732 42.8  8 20.380404.44050 1.85808 47.3  9 −10.01290 0.20000 10 −9.34050 0.70000 1.6200436.3 11* −1662.68660 0.46030 12 63.75380 3.51740 1.95000 45.0 13−17.45770 BF Image surface ∞ Aspherical data Surface No. Parameters  4 K= 0.00000E+00, A4 = 7.83179E−07, A6 = −4.38960E−07, A8 = 0.00000E+00 11K = 0.00000E+00, A4 = 1.07095E−04, A6 = −6.69901E−08, A8 = −2.26973E−09Various data Focal length 20.9999 F-number 1.75868 View angle 27.6727Image height 10.6000 Overall length of lens system 40.0031 BF 16.50214

(Numerical Example 7)

TABLE 7 Surface data Surface number r d nd vd Object surface ∞  115.66450 3.41190 1.88300 40.8  2 95.59600 0.20000 1.56732 42.8  395.59600 1.00000 1.51226 57.6  4 8.42640 4.99870  5(Diaphragm) ∞ 4.22570 6 −6.52430 0.70000 1.84065 23.9  7 −368.55240 0.01000 1.56732 42.8  8−368.55240 2.68300 1.88300 40.8  9 −9.83880 0.20000 10 ∞ 0.01000 1196.05340 2.54600 1.72916 54.7 12* −20.56080 1.28630 13 −34.18860 2.885801.72916 54.7 14 −14.97950 BF Image surface ∞ Aspherical data Surface No.Parameters 12 K = 0.00000E+00, A4 = 6.98520E−05, A6 = 1.77801E−07, A8 =−5.03014E−10 Various data Focal length 17.2940 F-number 1.83161 Viewangle 32.5142 Image height 10.1000 Overall length of lens system 41.5122BF 17.35485

(Numerical Example 8)

TABLE 8 Surface data Surface number r d nd vd Object surface ∞  139.62840 3.81320 1.89196 36.0  2 −1570.55980 0.29380  3 −349.344101.00000 1.48749 70.4  4 16.79960 12.61850  5(Diaphragm) ∞ 6.15460  6−8.19860 0.70000 1.76201 26.4  7 52.34840 0.01000 1.56732 42.8  852.34840 3.96030 1.88300 40.8  9 −12.53970 0.20000 10 22.39680 5.634601.88300 40.8 11 −25.79740 0.01000 1.56732 42.8 12 −25.79740 0.700001.65766 32.1 13 23.32640 1.72090 14* 32.69430 3.18420 1.72916 54.7 15*−43.55490 BF Image surface ∞ Aspherical data Surface No. Parameters 14 K= 0.00000E+00, A4 = −2.09332E−05, A6 = −2.18855E−07, A8 = 9.61996E−10 15K = −1.00000E+00, A4 = 4.01307E−05, A6 = −2.61765E−07, A8 = 2.20310E−09Various data Focal length 16.1935 F-number 1.69591 View angle 34.3196Image height 10.0000 Overall length of lens system 56.5096 BF 16.50953

(Numerical Example 9)

TABLE 9 Surface data Surface number r d nd vd Object surface ∞  123.74310 1.86710 1.84666 23.8  2 38.09010 0.20000  3 21.22650 1.000001.51788 70.1  4* 9.18330 7.62850  5(Diaphragm) ∞ 4.07000  6 −7.795700.70000 1.72969 27.8  7 194.37380 0.01000 1.56732 42.8  8 194.373802.74180 1.88300 40.8  9 −11.57520 0.20000 10 20.38860 4.55010 1.8830040.8 11 −14.83610 0.01000 1.56732 42.8 12 −14.83610 0.70000 1.68612 30.113 18.73850 1.24260 14 28.12240 2.85510 1.68863 52.9 15* −29.02610 BFImage surface ∞ Aspherical data Surface No. Parameters  4 K =0.00000E+00, A4 = 1.32817E−05, A6 = 1.06455E−07, A8 = 2.51890E−09 15 K =−1.00000E+00, A4 = 9.17495E−05, A6 = 3.98302E−09, A8 = 4.41564E−09Various data Focal length 14.4199 F-number 1.75937 View angle 37.6445Image height 9.7000 Overall length of lens system 43.9949 BF 16.21975

(Numerical Example 10)

TABLE 10 Surface data Surface number r d nd vd Object surface ∞  134.57820 3.28870 1.87384 27.0  2 71.89870 0.20000  3 55.92740 1.000001.51788 70.1  4 15.47950 17.14610  5(Diaphragm) ∞ 4.07000  6 −8.170800.70000 1.74472 27.1  7 24.94350 0.01000 1.56732 42.8  8 24.943503.44660 1.88300 40.8  9 −13.33380 0.20000 10 17.94210 3.97720 1.8830040.8 11 −23.80310 0.01000 1.56732 42.8 12 −23.80310 0.70000 1.63190 34.213 15.62420 1.40780 14 23.39480 2.61890 1.68863 52.9 15* −43.81450 BFImage surface ∞ Aspherical data Surface No. Parameters 15 K =−1.00000E+00, A4 = 1.10570E−04, A6 = −6.27408E−08, A8 = 5.37876E−09Various data Focal length 14.4199 F-number 1.75902 View angle 37.5518Image height 9.8000 Overall length of lens system 55.0013 BF 16.22601

(Numerical Example 11)

TABLE 11 Surface data Surface number r d nd vd Object surface ∞  126.37580 2.20370 1.88300 40.8  2 52.25560 0.20000  3 36.86130 1.000001.49710 81.6  4* 10.48480 9.03040  5(Diaphragm) ∞ 6.43500  6 −11.349500.70000 1.49461 28.1  7 149.09430 0.01000 1.56732 42.8  8 149.094303.02020 1.93953 45.2  9 −20.95220 0.20000 10 24.53320 4.98590 1.8740646.8 11 −28.93020 0.01000 1.56732 42.8 12 −28.93020 0.70000 1.45242 27.213 16.56430 2.31260 14* 35.28170 2.69200 1.58065 63.0 15* −38.95790 BFImage surface ∞ Aspherical data Surface No. Parameters  4 K =0.00000E+00, A4 = 2.71431E−05, A6 = −5.53610E−07, A8 = 2.21037E−08, A10= −2.31008E−10, A12 = 0.00000E+00, A14 = 1.78865E−14 14 K = 0.00000E+00,A4 = 1.34513E−05, A6 = −1.45457E−07, A8 = 1.79969E−09, A10 =2.18279E−11, A12 = −4.62803E−13, A14 = 0.00000E+00 15 K = 1.32446E+01,A4 = 1.18483E−04, A6 = −3.38852E−07, A8 = 1.29101E−08, A10 =−5.93769E−11, A12 = −3.58767E−13, A14 = 3.42944E−15 Various data Focallength 16.4786 F-number 1.43339 View angle 33.9723 Image height 10.1000Overall length of lens system 49.9971 BF 16.49734

The following Table 12 shows the values corresponding to the conditions(1) to (6) in the lens systems of the respective numerical examples.

TABLE 12 (Corresponding Values to Individual Conditions) ConditionsExamples (1) (2) (3) (4) (5) (6) 1 0.120 3.297 1.89292 0.375 −6.9%−0.082 2 0.124 3.318 1.89400 0.376 −7.5% −0.117 3 0.115 2.416 1.846660.359 −7.8% −0.083 4 0.198 2.244 1.89635 0.320 −4.8% 0.249 5 0.162 2.4171.72916 0.242 −7.8% −0.068 6 0.205 2.220 1.95000 0.261 −3.8% 0.307 70.191 2.397 1.88300 0.371 −8.3% 0.015 8 0.128 4.016 1.89196 0.404 −9.4%−0.087 9 0.110 2.887 1.84666 0.329 −12.5% −0.218 10 0.116 3.980 1.873840.356 −11.4% −0.128 11 0.102 3.343 1.88300 0.386 −8.9% −0.235

The lens system according to the present invention is applicable to animaging optical system. In particular, the lens system is suitable foran imaging optical system that is applicable to an interchangeable lensor a digital still camera.

While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It isunderstood that numerous other modifications and variations can bedevised without departing from the scope of the invention.

What is claimed is:
 1. A lens system having an F number equal to orsmaller than 2.4, wherein a positive lens element is disposed closest toan object side; a diaphragm disposed in a widest air space in the lenssystem; in a rear unit including all lens elements disposed on the imageside relative to the diaphragm, a lens element having a lens surface ofa smallest curvature radius is a negative lens element, and a lenselement having a lens surface of a second smallest curvature radius is apositive lens element; the negative lens element and the positive lenselement in the rear unit are adjacent to each other; and the lens systemsatisfies the following conditions:0.05<L _(—)1/L _(—) TH<0.211.5<L _(—) TH/Y<8 where, L_(—)1 is an interval from a lens surfacelocated closest to the object side to a lens surface located on theobject side relative to the diaphragm, L_TH is an interval from the lenssurface located closest to the object side to a lens surface locatedclosest to an image side, and Y is a maximum image height.
 2. The lenssystem according to claim 1, wherein a positive lens element and anegative lens element are disposed in order from the most object sidetoward the image side.
 3. The lens system according to claim 2satisfying the following condition:0.20<NdL1−NdL2<0.45 where, NdL1 is a refractive index of the positivelens element disposed closest to the object side, and NdL2 is arefractive index of the negative lens element disposed second closest tothe object side.
 4. The lens system according to claim 1, wherein two ormore positive lens elements are disposed in a rear unit including alllens elements disposed on the image side relative to the diaphragm. 5.The lens system according to claim 1, wherein in a rear unit includingall lens elements disposed on the image side relative to the diaphragm,an aspheric surface is provided on a lens element closest to the imageside or on a lens element second closest to the image side.
 6. The lenssystem according to claim 1 satisfying the following condition:1.70<NdL1<2.4 where NdL1 is a refractive index of the positive lenselement disposed closest to the object side.
 7. The lens systemaccording to claim 1 satisfying the following condition:−16%<Dist.<0% where Dist. is a distortion at the maximum image height.8. An interchangeable lens apparatus which allows the lens systemaccording to claim 1 to be attached to a camera body via an attachingpart.
 9. A camera system including the interchangeable lens apparatusaccording to claim 8, and an image sensor.
 10. A lens system having an Fnumber equal to or smaller than 2.4, wherein a diaphragm is disposed ina widest air space in the lens system; two or more lens elements eachhas a refractive index equal to or greater than 1.85; a positive lenselement and a negative lens element are disposed in order from the mostobject side toward the image side; in a rear unit including all lenselements disposed on the image side relative to the diaphragm, a lenselement having a lens surface of a smallest curvature radius is anegative lens element, and a lens element having a lens surface of asecond smallest curvature radius is a positive lens element; thenegative lens element and the positive lens element in the rear unit areadjacent to each other; and the lens system satisfies the followingconditions:1.5<L _(—) TH/Y<80.20<NdL1−NdL2<0.45 where, L_TH is an interval from a lens surfacelocated closest to an object side to a lens surface located closest toan image side, Y is a maximum image height NdL1 is a refractive index ofthe positive lens element disposed closest to the object side, and NdL2is a refractive index of the negative lens disposed second closest tothe object side.
 11. The lens system according to claim 10, wherein twoor more positive lens elements are disposed in a rear unit including alllens elements disposed on the image side relative to the diaphragm. 12.The lens system according to claim 10, wherein in a rear unit includingall lens elements disposed on the image side relative to the diaphragm,an aspheric surface is provided on a lens element closest to the imageside or on a lens element second closest to the image side.
 13. The lenssystem according to claim 10 satisfying the following condition:1.70<NdL1<2.4 where NdL1 is a refractive index of the positive lenselement disposed closest to the object side.
 14. The lens systemaccording to claim 10 satisfying the following condition:−16%<Dist.<0% where Dist. is a distortion at the maximum image height.15. An interchangeable lens apparatus which allows the lens systemaccording to claim 10 to be attached to a camera body via an attachingpart.
 16. A camera system including the interchangeable lens apparatusaccording to claim 15, and an image sensor.